The present invention relates to a process for the removal of dissolved metals and/or metalloids, such as As, Cd, Zn, Cut V, Ni, Sr, Sn, Ag, Se, Ba and Pb, from an aqueous medium, such as waste water, containing same and having a high content of salts, such as sodium, calcium and magnesium chlorides and sulphates.
Waste water formed by purification of flue gases from power plants and refuse incineration plants and stemming from deposits of solid waste in marine environment as well as fillings typically contain one or more of the above salts in concentrations which far exceed official requirements on drinking water, as well as dissolved metals and metalloids.
It is known to remove such dissolved metals and metalloids from waste water by chemical precipitation, e.g., by sulphide or hydroxide precipitation, with formation of large amounts of sludge, the separation of which from the waste water is cumbersome and cost consuming.
EP application No. 93919050.0 discloses a process for the removal of dissolved metals and in particular heavy metals from aqueous media containing same by passing the aqueous medium through a layer of a particulate carrier material in the presence of ferro ions and an oxidation agent and at such conditions that the carrier material particles are fluidized in the aqueous medium. Hereby coatings of amorphous ferric oxyhydroxide/ferric hydroxide are formed on the surfaces of the carrier material particles, to which coatings the heavy metals are bound by adsorption.
V. Janda and L. Bensova: "Removal of Manganese from Water in fluidized bed", Aqua, No. 16, pp. 313, 1988, discloses a process for the removal of Mn(II) from drinking water by oxidation with KMnO.sub.4 so as to form a MnO.sub.2 coating on the surface of the fluidized carrier material particles.
It is known, cf. e.g. T. C. Christensen and P. B. Nielsen: "Rensning af grundvand for tungmetaller", ATV-m.o slashed.de, Vinterm.o slashed.de om grundvandsforurening, Mar. 5-6, 1996, to combine a process for the removal of manganese(II) from drinking water as mentioned above with a removal of dissolved nickel.
Since drinking water normally only contains minor amounts of the salts mentioned in the preamble, the salts present in the drinking water will not interfere with the formation of a coating on the surfaces of the carrier material particles.
When removing dissolved metals and metalloids from aqueous media containing large amounts of salts, which herein should be taken to mean amounts of salt corresponding to salt concentrations for one or more of the salts of more than 1 g/l, the processes mentioned above are not readily applicable, since competition arises between the salts, e.g. sulphates, and the metals for the space necessary for the adsorption on the surfaces of the carrier material particles, which manifests itself in unsatisfactory removal of both manganese and dissolved metals and/or metalloids,
On closer studies of the adsorption and flow conditions in connection with the fluidization of carrier material particles in an aqueous medium containing Mn(II) ions and an oxidation agent, it has turned out that by using a carrier material having a density within given limits and by adapting the concentration of Mn(II) ions to the average grain diameter of the carrier material at the start of the process and the flow rate of the aqueous medium, sufficient space is obtainable on the surfaces of the carrier material particles for the salts in the aqueous medium not to block adsorption of manganese as well as metals or metalloids on these surfaces, simultaneously with it being avoided that the coatings which are formed on the particles are knocked off as a consequence of the particles colliding with each other.